This invention generally relates to latching devices, and is specifically concerned with a latching device for securing and sealingly engaging a closure around an opening in a cask used for transporting radioactive materials.
Devices for securing closures over the opening in a cask used for transporting radioactive materials are known in the prior art. In one of the most common prior art designs, a circular, lid-type closure is provided with thirty-six uniformly spaced bolt holes around its outer edge. These bolt holes are in turn registrable with threaded bores present in a ledge provided around a wall that circumscribes the opening in the cask. An elastomeric or metal O-ring is provided between the ledge and the closure to effect a gas-tight seal when the closure is mounted over the opening in the cask. In operation, the closure is placed over the opening in the cask so that its outer edge seats over the circular ledge and around the O-ring. The closure is then rotated so that the bolt holes around its outer edge are positioned into registry with the threaded bores present in the ledge. Stainless steel bolts are next inserted in opposing pairs of the bolt holes in the closure, and both bolts of each pair are simultaneously wrung up with a torque wrench until a desired compression between the closure and the ledge is achieved. The simultaneous wringing up of opposing bolts serves to uniformly compress the O-ring sandwiched between the closure and the ledge of the cask which in turn helps to form a uniform sealing engagement between the O-ring, the closure and the upper edge of the cask. Since Nuclear Regulatory Commission (NRC) regulations require such transportation casks to successfully contain radioactive gases and inert gas such as helium which may be under pressure, the total amount of compressive load that the bolts must apply between the closure and the ledge of the cask is on the order of about 500,000 lbsf. (or about 2.224.times.10.sup.6 nt). Consequently, the amount of torque which must be applied to each of the thirty-six bolts is considerable. Because these same NRC regulations require the closure to maintain its integrity with the cask upon falling a height of nine meters onto a hard surface, it is easy to see that the tensile and shear load requirements for each of the thirty-six bolts is considerable.
While bolt-type closing devices are capable of fulfilling the criteria set forth by the aforementioned NRC regulations, the applicant has observed a number of areas in the design of such prior art devices where improvement would be desirable. For example, the simultaneous application of large torque forces to eighteen pairs of opposing bolts takes a considerable amount of time, which in turn results in the exposure of the cask handlers to significant amounts of potentially harmful radiation. Still another deficiency in this design is the difficulty by which repairs are made in the event that one of more of the threaded bores in the cask becomes stripped through wear. When such stripping occurs, it may be necessary to re-drill the stripped bores and re-tap them so that they can accept a bolt having threads of larger outside diameter. Unfortunately, such a repair will mean that the closure can fit over the cask in only one specific angular position, i.e., the position where the larger bolt in the closure is in registry with the larger bolt hole. Of course, this "single orientation" problem could be remedied by merely reaming out all of the bolt holes and replacing all of the bolts with bolts having larger outside threads. But, such a repair effort would be time consuming and relatively costly. A third deficiency is the fact that, despite the use of large diameter bolts, these bolts still constitute the weakest part of such transportation casks. Hence, if the cask is subjected to the type of severe mechanical stresses that can be expected under accident conditions, the most likely area of failure is precisely the one that could cause the most damage--the area of attachment between the closure and the cask.
To overcome these deficiencies, it has been proposed that bolt-type closing devices be replaced with a "bank-door" hatch-type cover such as that disclosed and claimed in U.S. Pat. No. 4,519,519 by Robert E. Meuschke assigned to the Westinghouse Electric Corporation. In this device, a plurality of radially movable latches secure the hatch cover in place when a centrally disposed handwheel is rotated. Such a closing device is far faster to operate since the latches that it uses are all simultaneously extended or retracted by the rotation of a single handwheel. The design of this type of closing device is not, however, readily adaptable to a closure for a cask for transporting radioactive materials, since the various latches and their associated linkages are mounted on an exterior wall of the hatch, where they would be exposed to mechanical shock and possible breakage if the cask were dropped. Of course, a design could be envisioned wherein the latches and their linkages could be installed in the interior of a lid-type closure. But such a design would require the provision of slots and other cavities within the closure which would reduce the radiation shielding effectiveness of the closure. Still another difficulty in adapting such a design to a transportation cask is the fact that the centrally-disposed handwheel mechanism does not quite apply the same extension force to all of the latch elements simultaneously, or even symmetrically. This characteristic of "bank-door" type designs normally causes no problems in applications where the only purpose of the closing device is to lock a closure over an opening. But in applications where the closing device must also uniformly and sealingly engage a closure around the edge of an opening by applying a 500,000 lb. compressive load therebetween, such a non-symmetrical loading of the closure could interfere with the effectiveness of the metallic or elastomeric O-ring in sealing pressurized gases in the interior of the cask from the ambient atmosphere.
Clearly, there is a need for a device for removably securing and sealingly engaging a closure around an opening in a cask for transporting radioactive wastes that is faster to operate than prior art closing devices. Ideally, such a closing device should not interfere with the shielding effectiveness of the door, and should be able to withstand the nine meter drop requirements set forth in NRC regulations without any chance of breakage or rupture. Such a closing device should have parts which are easily and individually replaceable in the event of wear so that large portions of the entire cask need not be remachined during its service life. Finally, the closing device should be capable of applying uniform or at least symmetrical pressure around the closure during the closing operation to insure the effectiveness of the O-ring seal.